Biogas production from crop residues and aquatic weeds
Biogas generation and changes in total and volatile solids, total and NH~ -N, available P, and numbers of cellulolytic and acid-producing bacteria were monitored during a 120-day fermenta- tion period, in laboratory biogas digesters, of maize stalks, rice straw, cotton stalks, and water hyacinth, each enriched with partially digested cattle dung. Maize stalks gave the greatest values of cumulative yield of biogas (65 L/2.5 L fermented material) and its methane component (39 L), whereas rice straw, water hyacinth, and cotton stalks produced progressively less biogas. Such a sequence was paralleled by the rate of loss of volatile solids. Peaks of biogas generation appeared within the 17th to 21st day of fermentation, according to the type of feedstock. Appre- ciable amounts of biogas were produced up to fifty days of fermentation. Maize stalks, rice straw, and water hyacinth followed, respectively. The level of available phosphorus released showed the order: water hyacinth > cotton stalks > maize stalks > rice straw. Peak numbers of cellulose decom- posers appeared between the 10th and 20th day and those of the acid formers on the 20th day. Highest bacterial figures were shown by maize and cotton stalks for the cellulose decomposers, and by maize stalks and water hyacinth for the acid formers.